Below is a plot of some noise measurements on the PDA255 with which we have replaced the old PDA10A as the primary REFL diode. The traces are:
I realized that last time I did this I simply turned off the oscillator to take the no-EOM data. This is wrong because it also turns off the LO to the mixer, so the PD signal is not getting mixed down. This time, I physically disconnected the EOM to take this trace.
The plot is not very illuminating, except in that it shows that the noise in the demodulation setup is dominated by the dark noise of the PD. As seen in a previous post, the dark PD noise level is above but comparable to the input noise of the PDH box above ~1Hz. Below this---in our frequency band of highest interest---the PDH input noise dominates at the moment. The same linked plot shows that the theoretical noise estimate for the PDH input is substantially lower than the measured noise at low frequencies, so the PDs will certainly limit us if we get it to this level.
It goes without saying that both the final PDs and the final servo should contribute much less noise than these do.